1. Field of the Invention
The present invention pertains to a seal that contacts a portion of a patient to provide a comfortable interface between an external device, such as a respiratory mask, and the patient. Specifically, the present invention pertains to a seal having an elastic casing filled with a soft gel substance. The present invention also pertains to a respiratory mask having such a seal and to a method of interfacing a patient with an external device, such as a respiratory mask, using such a seal.
2. Description of Related Art
A variety of respiratory masks are known having a flexible seal that covers the areas surrounding the nose and/or mouth of a human user and that are designed to create a continuous seal against the user's face. Because of the sealing effect created, gases can be provided at a positive pressure within the mask for consumption by the user. The uses for such masks range from high altitude breathing (aviation applications), swimming, mining, and fire fighting applications and various medical diagnostic and therapeutic applications.
One requisite of many of these masks, particularly medical respiratory masks, is that they provide an effective seal against the user's face to prevent leakage of the gas being supplied. Commonly, in conventional mask configurations, a good mask-to-face seal has been attained in many instances only with considerable discomfort for the user. This problem is most crucial in those applications, especially medical applications, which require the user to wear the mask continuously for hours or perhaps even days. In such situations, the user will not tolerate the mask for long durations and optimum therapeutic or diagnostic objectives will not be achieved, or will be achieved with great difficulty and considerable user discomfort.
Several types of respiratory masks for the types of applications mentioned above are known. Perhaps the most common type of mask incorporates a smooth sealing surface extending around the periphery of the mask and exhibiting a generally uniform, i.e., predetermined or fixed, seal surface contour that is intended to be effective to seal against the user's face when force is applied to the mask with the sealing surface in confronting engagement with the user's face. The sealing surface typically consists of an air or fluid filled cushion, or it may simply be a molded or formed surface of a resilient seal element made of an elastomer such as plastic, rubber, silicone, vinyl or foam.
Such masks have performed well when the fit is good between the contours of the seal surface and the corresponding contours of the user's face. This may occur, for example, if the contours of the user's face happen to match well with the predetermined contours of the seal. However, if the seal fit is not good, there will be gaps in the seal-to-face interface resulting in gas leaking from the mask at the gaps. Excessive force will be required to compress the seal member to close the gaps and attain a satisfactory seal in those areas where the gaps occur. Such excessive force is unacceptable because it produces high pressure points elsewhere on the face of the user where the mask seal contour is forcibly deformed against the face to conform to the user's facial contours. This will produce considerable user discomfort and possible skin irritation and breakdown anywhere the applied force exceeds the local perfusion pressure, which is the pressure that is sufficient to cut off surface blood flow. Ideally, contact forces should be limited between the mask and the user's face to avoid exceeding the local perfusion pressure, even at points where the mask seal must deform considerably.
The problem of seal contact force exceeding desirable limits is even more pronounced when the positive pressure of the gas being supplied is relatively high or is cyclical to relatively high levels. Because the mask seals by virtue of confronting contact between the mask seal and the user's face, the mask must be held against the face with a force sufficient to seal against leakage of the peak pressure of the supplied gas. Thus, for conventional masks, when the supply pressure is high, headstraps or other mask restraints must be relatively tightly fastened. This produces high localized pressure on the face, not only in the zone of the mask seal, but at various locations along the extent of the retention straps as well. This, too, will result in discomfort for the user after only a brief time. Even in the absence of excessive localized pressure points, the tight mask and headstraps may become extremely uncomfortable, and user discomfort may well cause discontinued cooperation with the treatment regimen. Examples of respiratory masks possessing continuous cushion sealing characteristics of the type just described are provided in U.S. Pat. Nos. 2,254,854 and 2,931,356.
U.S. Pat. No. 5,181,506 describes a protective gas mask for military applications. The mask includes a three-layer face piece, the central layer of which is a thick layer of relatively stiff material having preformed V-shaped channels. The channels are “overfilled” with a gel or both gel and compressed air to create bulges in an inner face-contacting layer that are adapted to seal against the contours of a user's face. The inherent stiffness of the central layer in combination with the structural rigidity provided by the V-shaped channels, especially when overfilled with gel/air, results in a comparatively unyielding facial seal. Indeed, the mask is deployed in combination with a tightly fitting hood in order to draw the face piece firmly against the user's head to generate the desired facial seal. As will be appreciated, the comfort afforded such a construction is quite limited and certainly not appropriate for those applications, such as respiratory therapy situations, where a user must occasionally wear a mask for prolonged periods of time.
Several classes of cushion materials, including gels and foams, were analyzed in a study by S. F. C. Stewart, V. Palmieri and G. V. B. Cochran, Arch. Phys. Med. Rehabil., Vol. 61, (May 1980). That study compared the relative advantages and disadvantages of such cushion materials when used as wheelchair cushions, specifically the effects of such materials on skin temperature, heat flux, and relative humidity at the skin-cushion interface. Each of these factors, along with applied pressure in excess of local perfusion pressure, has been identified as a contributor to breakdown of skin tissue at the skin-cushion interface.
In that study, foam cushions were reported to increase skin temperatures by several degrees after a few hours of use. This was suggested to be a result of the comparatively low heat flux characteristics of foam materials. That is, the foam materials and the air entrapped within them tend to be poor conductors of heat. Conversely, gel pads, as a group, showed a considerably higher heat flux than foam, sufficient, in fact, to maintain skin temperatures relatively constant after several hours of use. The sole benefit of foam versus gel reported in the study was that foams produced lesser relative humidity than gels at the skin-cushion interface. This was attributed to the open cell structure of the foams which provide a pathway through which moisture can diffuse. This seeming advantage is somewhat problematic, however, in that open cell foam tends to promote bacteria growth when exposed to perspiration. Bacteria, in turn, contaminate the foam thereby considerably hindering its useful service life.
Moreover, whether air, fluid or, in the case of U.S. Pat. No. 5,181,506, gel filled, or whether formed as an elastomer such as foam, plastic, rubber, silicone and the like, the resiliency or recoil characteristics of presently available cushion type respiratory mask seals have not been well suited to form an effective seal with the topography of the user's face in the absence of considerable headstrap tensile forces.
A respiratory mask facial seal comprising a seal cushion formed of a gel substance is disclosed in U.S. Pat. Nos. 5,647,357 and 5,884,624, the disclosures of which are herein incorporated by reference. The gel substance is a viscoelastic polyurethane polymer possessing resilience or recoil characteristics corresponding substantially to those of human fat tissue. Specifically, the seal cushion has a resiliency, as defined by durometer measured on the Shore 00 scale which is used to gauge the resiliency of very soft resilient materials, of about 10 or softer and, most preferably, about 0. Such resiliency corresponds substantially to that of human fat tissue which also exhibits a durometer reading of 0 on a Shore 00 scale. More specifically, the seal cushion exhibits a resiliency or durometer on the Shore 000 scale (which scale is used to measure the resiliency of extremely soft resilient materials) of about 20 to about 45. By comparison, human fat tissue registers a durometer of about 10 on the Shore 000 scale. In one embodiment, the gel substance is covered by a flexible plastic film.
A customizable seal that contacts a portion of a patient is disclosed in U.S. Pat. Nos. 6,397,847 and 6,895,965, the disclosures of which are herein incorporated by reference. The customizable seal, in a preferred embodiment, has a first portion fabricated from a gel substance having the recoil characteristic analogous to that of human fat, as disclosed in U.S. Pat. Nos. 5,647,357 and 5,884,624. The seal has a second portion associated with the first portion and including a selectively formable substance adapted to be molded from a first pattern into a second pattern and to retain the second pattern responsive to being so molded.
Some of the known conventional gel masks discussed above include a non-elastic casing encapsulating a gel substance. The casing is formed from a polyurethane which has a typical hardness of 75 Shore A and 80-250% elongation. This non-elastic casing is thermal formed from very thin film (approximately 2 to 10 mils thick). The forming capability of such a thin film limits the complexity of the geometry and the wall thickness distribution. Also, because of the thinness of the film, the encapsulation provides no structural function. The gel substance, therefore, has to provide the form and structure for the seal and is relatively hard (ranging from 20 to 25 Shore 00).